With the present shortage of gasoline available for operation of automobiles, it is becoming increasingly important that internal combustion engines and specifically those which drive automobiles and trucks be operated at optimum efficiency, i.e. with maximum reasonably obtainable miles per gallon. It has long been known that the efficiency of an engine is directly, although not necessarily linearly, related to the vacuum in the intake manifold at any given moment. Thus many sophisticated drivers have taken to installing a manifold vacuum gauge on the dash instrument panel of their automobile which they observe closely. As long as the vacuum indicated is above a certain minimum value, typically around 10 to 12 inches of mercury, the driver knows that (with a typical engine) he is operating in a range of good gas economy. When the vacuum needle drops below that point, he knows that his gas mileage has, at that instant of operation, dropped off markedly.